The invention relates to a cathode ray tube having a longitudinal axis, a phosphor screen, an electron gun arranged around the longitudinal axis, the electron gun comprising a triode part having three cathodes for generating a red, a green and a blue electron beam, respectively, and two common grids arranged transversely to the longitudinal axis, and a focus lens part having at least two common grids arranged transversely to the longitudinal axis.
Normally, a cathode ray tube for television or other purposes has an envelope with a large bulb portion and a tubular neck portion fixed thereto. A plurality of electrodes for forming and focusing electron beams along a path extending into the bulb portion is mounted within the tubular neck portion. Magnetic or electrostatic field-producing means are used for moving the electron beams in any desired manner over a wall portion or face plate of the envelope bulb portion. The inner surface of the wall or face plate is coated with a film of phosphor material which luminesces with a visible light when struck by the electron beams. By modulating the current of the electron beams, the scanned area of the phosphor screen can be varied in a manner to produce a light path in accordance with modulating signals applied to the electron gun of the tube.
When using a CRT during the manufacture of, e.g. a television set or a monitor, the setmaker has to set the white point. It turns out, however, that the white point of the finalized apparatus is sometimes shifted and displayed images become discolored.
It is an object of the present invention to provide a cathode ray tube which allows a display to have a substantially stable white point during its life.
The cathode ray tube in accordance with the invention is characterized in that the cathode for generating the green electron beam is offset from the longitudinal axis, the electron gun being capable of having the green electron beam in the center of the phosphor screen.
The invention is based on the recognition that initially, during the adjustments of the cathode ray tube apparatus, the xe2x80x9cgreenxe2x80x9d cathode has a temporary loss of emission due to residual gases being present inside the tube and due to ionization of the gas atoms leading to bombardment of the cathode surfaces with positive ions. The emission improves substantially after one or two hours of tube operation (scanning aging or raster aging, also called the soak test). As a result, the white point is shifted and displayed images become greener.
The reason why the xe2x80x9cgreenxe2x80x9d cathode suffers more severely from loss of emission than the xe2x80x9credxe2x80x9d and xe2x80x9cbluexe2x80x9d cathodes can be explained by the presence of kinks in the xe2x80x9credxe2x80x9d and xe2x80x9cbluexe2x80x9d electron beams in the neighborhood of grid G3a, necessary for convergence of the three beams. For reasons of symmetry (concerning electron guns with in-line cathodes), such a kink is not present in the xe2x80x9cgreenxe2x80x9d electron beam.
The unequal amounts of emission loss for the xe2x80x9credxe2x80x9d and xe2x80x9cbluexe2x80x9d cathodes, on the one hand, and the xe2x80x9cgreenxe2x80x9d cathode, on the other hand, lead to discoloration of the displayed images. Therefore, the object of this invention is to obtain a new electron gun design for which the three cathodes are equally exposed to ion bombardment and thus suffer from emission loss with equal severity. This can be achieved by introducing a kink in the xe2x80x9cgreenxe2x80x9d electron beam too.
This kink can be constructed analogously to the kinks in the xe2x80x9credxe2x80x9d and xe2x80x9cbluexe2x80x9d electron beams in either the horizontal direction or the vertical direction. This idea is worked out below for the cathode positions and for the positions of the grid apertures, where a slight offset from the longitudinal axis (z-axis), or from the original in-line plane (the x-z plane) is considered. A delta orientation of the three cathodes can also be considered.
The kink in the xe2x80x9cgreenxe2x80x9d beam requires an electron-gun design in which apertures are made for the green beam in the G2 and/or in the G3a-grid, which are eccentric with respect to the apertures in the G1 grid.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.